Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface, showing my new design and depicting a first image of the transitional graphical user interface.

FIG. 2 is a front view thereof, depicting a second image of the transitional graphical user interface; and,

FIG. 3 is a front view thereof, depictinga third image of the transitional graphical user interface.

The appearance of the transitional graphical user interface sequentially transitions among the images shown in FIGS. 1-3. The process or period in which one image transitions to another image forms no part of the claimed design.

The broken lines showing the device and display screen illustrate environmental structure and portions of the article. All other broken lines show portions of the transitional graphical user interface. The broken lines form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 